The invention relates to an electrophotographic image forming apparatus such as copying machine, printer, facsimile, and so on of the electrophotographic type. Particularly, the invention relates to an image forming apparatus using toner that contains comparatively much wax.
A conventional electrophotographic image forming apparatus forms an image on a recording medium by the steps of charging the surface of an image carrier such as a photosensitive body with electricity, applying a laser beam to the electrically-charged surface in a scanning manner to form a latent image thereon, applying toner to the latent image by a developer to make it visible, heating, melting, and pressing the toner image by a fixing device to fix the toner image to a recording medium, and then ejecting the recording medium to the outside of the apparatus. The toner on the recording medium can be cooled and fixed thereto until the recording medium is ejected from the fixing device to the outside of the apparatus.
In case of forming a color image, toners of four colors (yellow, magenta, cyan, and black) are used. The image forming apparatus forms a toner image of each color on the related photosensitive body, transfers toner images of the colors onto an intermediate transfer body in a superposition manner, and further transfers the superimposed multi-color images to a recording medium. Therefore, a color image uses more toner than a black and white image. When the quantity of toner increases, the toner heated by the heating roller of the fixing device becomes harder to be separated from the heating roller. Then, wax is added to toner to make toners be separated from the heating roller easily. Wax in the toner can make color images glossy and consequently improve the image quality.
Usually, wax is set to be molten more easily than toner. Thus, wax is lower in melting and solidifying points than toner. Consequently toner is solidified immediately after it goes out from the fixing device, but wax is slow to be solidified.
The recording medium is carried by the ejection rollers to pass through an ejection path between the exit of the fixing device and the outside of the apparatus. This ejection path is not linear but curved with guide plates or the like so that the recording medium may travel a preset curved course before reaching the ejection roller.
When the recording medium touches the guide plates while wax is not solidified yet, the molten wax on the contact area of the recording medium is quickly cooled by it and solidified. However, wax on the other recording medium area is slow to be solidified.
Generally, wax contained in toner is glossy when the molten wax is cooled and solidified quickly but becomes dull when the molten wax is cooled gradually. Therefore, the wax on the contact area of the recording medium is immediately cooled and becomes glossy but the wax on the non-contact area of the recording medium is cooled slowly and becomes dull. The uneven glossiness on the recording medium reduces the image quality. To prevent this, the recording medium must be carried without being in contact with the guide plates or the like.
Patent Document 1, which represents Japanese Non-examined Patent Publication 2001-175112, discloses a method of transferring a recording medium without making the recording medium touch the guide plate or the like. In the description of Patent Document 1, the recording medium (a transfer sheet) passing through a nip area between the heating roller and the pressing roller of the fixing device is separated from the heating roller by separation claws provided near the heating roller. The transfer sheet is separated by the tip of each separation claw, slides on the surface of the claw to the ejection rollers, and then carried into the ejection tray. In this case, paper dust of the transfer sheet may remove the Teflon (registered trademark) coated layer. In extreme cases, toner may be caught in the area from which Teflon coat is removed and may give unwanted lines to the image on the transfer sheet or damage the transfer sheet. Patent Document 1 provides a spur on each separation claw so that the transfer sheet passing through the nip area between the heating roller and the pressing roller may not be in contact with the separation claws. Specifically, the transfer sheet separated from the heating roller by the separation claws is supported by the edges of the spurs away from the separation claws and sent to the ejection rollers. This mechanism keeps the transfer sheet away from the separation claws, prevents the Teflon coat from being damaged, and consequently prevents unwanted lines on the transfer sheet and damages of the transfer sheet.
Patent Document 2, which represents Japanese Non-examined Patent Publication H11-95489, as another prior art discloses a means provided in the ejection path between the image carrier and the fixing device. This means is to carry a transfer sheet having unfixed toner images on both sides. When the transfer sheet touches the guide plate or the like, the unfixed toner on the transfer sheet may be rubbed away by it. To prevent this, the invention provides spurs in the ejection path and supports the transfer sheet by a plurality of spur edges without carrying the transfer sheet on the guide plates. As the spurs rotate while the transfer sheet moves forward, the transfer sheet is supported by a plurality of spur edges and toner images on the transfer sheet will not be damaged.
Naturally, there are many parts other than the guide plates and the like that touch the molten wax on the transfer sheet. The transfer sheet is fed from the sheet cassette, carried along a preset conveyance path through the image forming apparatus, and finally ejected to the ejection tray. A plurality of sensors are disposed along this conveyance path. If a sensor does not detects the transfer sheet within a preset time period after the preceding sensor detected the transfer sheet, it is assumed that a paper jam has occurred. These sensors are helpful in detecting a paper jam earlier and enable easy and quick removal of the jam.
Each sensor has an actuator. When touching the leading edge of a transfer sheet, the actuator works to turn on the sensor to tell that the transfer sheet is detected. When the actuator touches molten wax on the transfer sheet, the wax is quickly cooled and solidified. This causes a glossy stripe on the transfer sheet.
However, in the above well-known example, the recording medium is kept away from the guide plates or the like before fixing or immediately after fixing. Contrarily, the molten wax is solidified near the ejection roller which is a little away from the fixing device. Therefore, this prior art cannot solve the unevenness of gloss of wax by a sensor provided just after the fixing device.